全文获取类型
收费全文 | 39456篇 |
免费 | 2734篇 |
国内免费 | 4024篇 |
专业分类
教育 | 24945篇 |
科学研究 | 7058篇 |
各国文化 | 298篇 |
体育 | 4242篇 |
综合类 | 1764篇 |
文化理论 | 234篇 |
信息传播 | 7673篇 |
出版年
2022年 | 236篇 |
2021年 | 338篇 |
2020年 | 671篇 |
2019年 | 1259篇 |
2018年 | 1786篇 |
2017年 | 1812篇 |
2016年 | 1727篇 |
2015年 | 1529篇 |
2014年 | 1659篇 |
2013年 | 6201篇 |
2012年 | 1980篇 |
2011年 | 2162篇 |
2010年 | 2017篇 |
2009年 | 1745篇 |
2008年 | 2127篇 |
2007年 | 1847篇 |
2006年 | 1001篇 |
2005年 | 984篇 |
2004年 | 1207篇 |
2003年 | 939篇 |
2002年 | 962篇 |
2001年 | 818篇 |
2000年 | 759篇 |
1999年 | 614篇 |
1998年 | 452篇 |
1997年 | 481篇 |
1996年 | 458篇 |
1995年 | 410篇 |
1994年 | 330篇 |
1993年 | 329篇 |
1992年 | 432篇 |
1991年 | 366篇 |
1990年 | 391篇 |
1989年 | 374篇 |
1988年 | 338篇 |
1987年 | 346篇 |
1986年 | 309篇 |
1985年 | 359篇 |
1984年 | 300篇 |
1983年 | 251篇 |
1982年 | 212篇 |
1981年 | 192篇 |
1980年 | 178篇 |
1979年 | 281篇 |
1978年 | 204篇 |
1977年 | 182篇 |
1976年 | 172篇 |
1975年 | 154篇 |
1974年 | 163篇 |
1973年 | 152篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
This article addresses file maintenance of subject headings in the National Library of Medicine's MEDLINE, a bibliographic retrieval file which is indexed using Medical Subject Headings (MeSH), NLM's subject authority. The emphasis is class maintenance in which sets of records in MEDLINE are changed by maintenance actions which reflect yearly changes in MeSH. Specific types of maintenance action are described with examples to highlight problem areas. Class maintenance failures, checking class maintenance, and informing end-users of MeSH changes resulting in class maintenance are discussed. Technical constraints and feasibility are not analyzed in detail; rather, the approach is geared toward requirements and impact of maintenance from the standpoint of vocabulary specialists and end-users. The article concludes with a brief statement on defining file maintenance policy. 相似文献
992.
The rate of citation duplication was examined in three databases: MEDLINE, BIOSIS, and LIFE SCIENCES COLLECTION. Duplicate citations were found to be more pertinent than unique citations. The duplicate citations came from a highly compact literature, while those from a single database were very widely scattered. The pertinent duplicated citations were more likely to be retrieved in searches that had more terms overall, had a higher percentage of thesaurus terms, and had terms which appeared in both title and abstract. These results suggest that the rate of duplication of citations in multidatabase searches may be used to rank output according to probable pertinence. 相似文献
993.
Proteins and glutathione (GSH) levels were estimated in the experimental groups. Levels of both proteins and glutathione in
the control and the treated groups were found to be significantly decreased compared to the normal group. The results show
that supplementation of riboflavin has beneficial effect by alleviating the oxidant injuries to GSH and proteins, which are
involved in the maintenance of transparency and protection of thiol groups of the lens. Thus, riboflavin has an active role
in the lens metabolism and has a potential to improve the metabolic activity of the cataractous lenses. 相似文献
994.
R Abraham M Joseph John R Calton J Dhanoa 《Indian journal of clinical biochemistry : IJCB》2006,21(1):95-100
Homocysteine(Hcy) has been implicated as a novel risk factor of Coronary Artery Disease (CAD) among Asian Indians, but many
studies done in India failed to reveal any direct correlation. It has also been reported that Folic acid and Vitamin B12 levels
inversely affect serum levels of homocysteine. In this study, we looked at the levels of homocysteine among patients with
CAD. The effect of Vitamin B12, Folate and other risk factors on homocysteine levels were also evaluated. Mean homocysteine
levels in cases (22.81±13.9, n=70) were significantly higher (p=<0.001) than the controls (7.77±7.3, n=70). However no statistically
significant correlation could be deduced between homocysteine Vitamin B12, and Folate. Cumulative analysis have indicated
an increase in homocysteine levels among patients with CAD with every additional risk factor. 相似文献
995.
Both structured and unstructured data, as well as structured data representing several different types of tuples, may be integrated into a single list for browsing or retrieval. Data may be arranged in the Gray code order of the features and metadata, producing optimal ordering for browsing. We provide several metrics for evaluating the performance of systems supporting browsing, given some constraints. Metadata and indexing terms are used for sorting keys and attributes for structured data, as well as for semi-structured or unstructured documents, images, media, etc. Economic and information theoretic models are suggested that enable the ordering to adapt to user preferences. Different relational structures and unstructured data may be integrated into a single, optimal ordering for browsing or for displaying tables in digital libraries, database management systems, or information retrieval systems. Adaptive displays of data are discussed. 相似文献
996.
【目的】实现自动提取科技期刊全文元数据并生成HTML文件。【方法】 以方正排版文件为对象,在可以提取出来文章的标题、摘要等元数据的基础上,将文章的正文内容元数据化,提出了包含图、表、公式等的广义元数据概念,并建立了提取图、表元数据的提取规则,同时将方正排版数学公式转化为LaTeX表达式。然后利用VB编程软件编写了自动提取广义元数据程序并将元数据重新组合生成HTML格式的文件。【结果】 根据方正BD排版语言的特点,建立的提取规则能有效提取全文并元数据化,最后可直接生成HTML文件。【结论】 实际应用表明了利用广义元数据生成HTML文件的有效性和可行性。 相似文献
997.
基于微信公众平台的科技期刊增值服务模式探析 总被引:1,自引:1,他引:1
【目的】构建基于微信公众平台的科技期刊增值服务模式。【方法】 对133种中华医学会系列杂志的微信公众平台进行抽样调研,筛选出2016年1至11月平均微信传播指数(WeChat Communication Index,WCI)排名前10位的期刊,分析其增值服务评估指标。【结果】月均WCI排名前10位期刊的增值服务表现平平,但也不乏亮点。【结论】亟需以优化用户体验为出发点,构建基于微信公众平台的科技期刊增值服务模式,以自助式服务提高信息的易得性,以交互式服务建立新型编读作关系,以主动式服务创造良好的用户体验,在为用户创造更多价值的同时不断积淀自身的品牌价值。 相似文献
998.
基于GIS的中国气候分区及综合评价 总被引:14,自引:1,他引:14
气候是生态环境的一个重要因子,也是生态环境评价中的重要指标。本文应用地理信息系统技术,基于全国1973年~2002年734个气象站点的基本气象数据,计算了各气象站的多年平均气温、≥10℃积温、干燥度、年降水量和年降水变率等气候指标;对每个气候指标,利用聚类分析,将全国分成4~7个区域,进而在每个分区里利用"回归分析 残差修正"的方法进行插值,生成各气候指标的1km×1km栅格数据,最后利用层次分析法(AHP)建立全国的气候综合评价模型,对我国气候进行了综合评价,实现了参考土壤、植被等其他空间数据、进行气候要素区域空间变异的定量化表达及综合分析与评价。 相似文献
999.
1000.
Knowledge Management Research & Practice - The definition and subsequent use of knowledge within and across organisational and social contexts has been a vibrant and evolving growth area over... 相似文献